1. Field of the Invention
The present invention relates to a photocatalyzer, and to a lamp and a lighting fixture using the photocatalyzer.
2. Description of Related Art
It is known to use a photocatalytic membrane for removing odor, preventing contamination and for antibacterial use. When a photocatalytic membrane receives and absorbs ultraviolet rays, photocatalytic action takes place. The membrane is ionized and generates electrons and holes. These electrons and holes react with oxygen and moisture present on the surface of the membrane, generate active oxygen and other active radicals, and oxidize and reduce constituents of contaminants that may cause odors, such as organic matter and the like.
At this time it appears that the most promising material that can be used on an industrial scale that exhibits photocatalytic action is titanium oxide. Titanium oxide has remarkable photcatalytic action and is available at reasonable prices in industrial quantity. Titanium oxide is available in two different crystal structures: rutile type and anatase type. The anatase type crystal of titanium oxide has excellent photocatalytic action.
Due to the perceived usefullness of a photocatalytic membrane, there have been efforts to form a photocatalytic membrane on a wide range of products such as building materials, lighting fixtures, lamps, etc. There are various known methods for manufacturing a photocatalytic membrane. Two such methods are the so-called dipping method and a method in which ultra fine particles are dispersed in a liquid used for coating.
The so-called dipping method forms a photocatalytic membrane by coating a base body with a liquid containing titanium alkoxide and baking it at 400.about.500.degree. C. The photocatalytic membrane obtained according to this method is durable because it has excellent membrane strength.
The ultra fine particle dispersed liquid coating method forms a photocatalytic membrane by applying a water soluble dispersed liquid with such photocatalytic ultra fine particles as titanium oxide dispersed in water and isopropyl alcohol to a base body and baking it. A photocatalytic membrane formed by this manufacturing method has a high crystallization and has excellent photocatalytic properties.
It is difficult to form a high quality photocatalytic membrane using the so-called dipping method. Unless it is incinerated at a high temperature for a long time, the crystallization of the surface is insufficient and its photocatalytic characteristic is low. When the base body is made of a soft glass such as a soda lime glass, the glass has a softening temperature that is relatively low. Thus, the glass cannot be incinerated at a sufficient temperature to achieve the desired photocatalytic characteristic.
When the photocatalytic membrane is formed according to the above-mentioned manufacturing method, there is also a problem in that the transmission factor for visible light is diminished. This is due to light interference action generated from a difference in refractive indexes of the glass and photocatalytic membrane. The refractive index of the photocatalytic membrane made of mainly titanium oxide is larger than that of glass.
In order to solve these problems associated with the dipping method, the applicant previously proposed a method to reduce a refractive factor of a photocatalytic membrane so that it is almost equal to that of glass, which is used as the base body. This was done by adding a metallic oxide such as silica in the photocatalytic membrane. The details of this attempt are set forth in Japanese Patent Application No. 9-140372, the subject matter of which is incorporated herein by reference as if fully set forth herein. As described in that patent application, it was possible to prevent the drop of the visible rays transmission factor by adding metallic oxide so as to lower the refractive index of the photocatalytic membrane and prevent the generation of interference color. However, an unfortunate side effect of this procedure was that the photocatalytic action also drops somewhat.
On the other hand, when the photocatalytic membrane is formed according to the so-called dipping method to attach a coating liquid to a base body by dipping it in a coating liquid containing such organic titanium compound as titanium alkoxide, titanium alkoxide and high boiling point organic compound causes hydrolysis with moisture in the air when pulling the base body out of the coating liquid. However, moisture does not readily arrive at the base body side and the hydrolysis at the base body side is insufficient and is not completely evaporated during the heating process at the time of calcination and is thermally cracked and scattered via such intermediate product as hydrocarbon. Therefore, it was revealed that impurities such as carbon increases in the photocatalytic membrane which tends to be reduced and titanium oxide in the photocatalytic membrane tends to become a rutile crystal that is partially inferior in the photocatalytic action. Thus the photocatalytic action of an obtained photocatalytic membrane tends to drop.
On the other hand, in the ultra fine particles dispersed liquid coating method, the obtained photocatalytic membrane generally has a high crystallization and excellent photocatalytic characteristic but there are problems. Adherence to the main body is not sufficient and when an organic bonding agent is used, this bonding agent tends to cause cracks. When the bonding agent cracks, the transmission factor may drop due to cloudiness.